Various methods have been proposed to solve the general depth-resolved fluorescence problem in optically-turbid media such as tissue. Point-detection methods have been developed, such as using the depth-dependent distortion of the fluorescence emission spectrum by tissue absorption (Swartling et al., 2005) or using spatially-resolved diffuse fluorescence to determine depth (Hyde et al., 2001).
Since there may be limitations to point detection methods in a surgical field, wide-field methods have also been pursued. One method has the diffuse fluorescence pattern imaged using broad-beam illumination, with the rather restrictive modeling assumption that the fluorescence source is point-like (Comsa et al., 2008). Laminar optical tomography has also been developed for full three-dimensional reconstruction, where a laser is raster-scanned over the tissue surface and the diffuse fluorescence pattern imaged at each xy point (Hillman et al., 2004; Kepshire et al., 2007). The major issue with optical tomography (i.e. full 3-D reconstructions) is that it is generally accepted to be an ill-posed problem (Arridge, 1999); as well, the data may be corrupted by uneven tissue surfaces or tissue movement during the long acquisition times.